Properties of a novel K+ current that is active at resting potential in rabbit pulmonary artery smooth muscle cells

Potassium (K+) channels in the pulmonary vasculature: Implications in pulmonary hypertension Physiological, pathophysiological and pharmacological regulation

The large K⁺ channel functional diversity in the pulmonary vasculature results from the multitude of genes expressed encoding K⁺ channels, alternative RNA splicing, the post-transcriptional modifications, the presence of homomeric or heteromeric assemblies of the pore-forming α-subunits and the existence of accessory β-subunits modulating the functional properties of the channel. K⁺ channels can also be regulated at multiple levels by different factors controlling channel activity, trafficking, recycling and degradation. The activity of these channels is the primary determinant of membrane potential (Em) in pulmonary artery smooth muscle cells (PASMC), providing an essential regulatory mechanism to dilate or contract pulmonary arteries (PA). K⁺ channels are also expressed in pulmonary artery endothelial cells (PAEC) where they control resting Em, Ca²⁺ entry and the production of different vasoactive factors. The activity of K⁺ channels is also important in regulating the population and phenotype of PASMC in the pulmonary vasculature, since they are involved in cell apoptosis, survival and proliferation. Notably, K⁺ channels play a major role in the development of pulmonary hypertension (PH). Impaired K⁺ channel activity in PH results from: 1) loss of function mutations, 2) downregulation of its expression, which involves transcription factors and microRNAs, or 3) decreased channel current as a result of increased vasoactive factors (e.g., hypoxia, 5-HT, endothelin-1 or thromboxane), exposure to drugs with channel-blocking properties, or by a reduction in factors that positively regulate K⁺ channel activity (e.g., NO and prostacyclin). Restoring K⁺ channel expression, its intracellular trafficking and the channel activity is an attractive therapeutic strategy in PH.

Vascular Kv7 channels control intracellular Ca2+ dynamics in smooth muscle

Voltage-gated Kv7 (or KCNQ) channels control activity of excitable cells, including vascular smooth muscle cells (VSMCs), by setting their resting membrane potential and controlling other excitability parameters. Excitation-contraction coupling in muscle cells is mediated by Ca2+ but until now, the exact role of Kv7 channels in cytosolic Ca2+ dynamics in VSMCs has not been fully elucidated. We utilised microfluorimetry to investigate the impact of Kv7 channel activity on intracellular Ca2+ levels and electrical activity of rat A7r5 VSMCs and primary human internal mammary artery (IMA) SMCs. Both, direct (XE991) and G protein coupled receptor mediated (vasopressin, AVP) Kv7 channel inhibition induced robust Ca2+ oscillations, which were significantly reduced in the presence of Kv7 channel activator, retigabine, L-type Ca2+ channel inhibitor, nifedipine, or T-type Ca2+ channel inhibitor, NNC 55-0396, in A7r5 cells. Membrane potential measured using FluoVolt exhibited a slow depolarisation followed by a burst of sharp spikes in response to XE991; spikes were temporally correlated with Ca2+ oscillations. Phospholipase C inhibitor (edelfosine) reduced AVP-induced, but not XE991-induced Ca2+ oscillations. AVP and XE991 induced a large increase of [Ca2+]i in human IMA, which was also attenuated with retigabine, nifedipine and NNC 55-0396. RT-PCR, immunohistochemistry and electrophysiology suggested that Kv7.5 was the predominant Kv7 subunit in both rat and human arterial SMCs; CACNA1C (Cav1.2; L-type) and CACNA1 G (Cav3.1; T-type) were the most abundant voltage-gated Ca2+ channel gene transcripts in both types of VSMCs. This study establishes Kv7 channels as key regulators of Ca2+ signalling in VSMCs with Kv7.5 playing a dominant role.

On a Magical Mystery Tour with 8-Bromo-Cyclic ADP-Ribose: From All-or-None Block to Nanojunctions and the Cell-Wide Web

A plethora of cellular functions are controlled by calcium signals, that are greatly coordinated by calcium release from intracellular stores, the principal component of which is the sarco/endooplasmic reticulum (S/ER). In 1997 it was generally accepted that activation of various G protein-coupled receptors facilitated inositol-1,4,5-trisphosphate (IP3) production, activation of IP3 receptors and thus calcium release from S/ER. Adding to this, it was evident that S/ER resident ryanodine receptors (RyRs) could support two opposing cellular functions by delivering either highly localised calcium signals, such as calcium sparks, or by carrying propagating, global calcium waves. Coincidentally, it was reported that RyRs in mammalian cardiac myocytes might be regulated by a novel calcium mobilising messenger, cyclic adenosine diphosphate-ribose (cADPR), that had recently been discovered by HC Lee in sea urchin eggs. A reputedly selective and competitive cADPR antagonist, 8-bromo-cADPR, had been developed and was made available to us. We used 8-bromo-cADPR to further explore our observation that S/ER calcium release via RyRs could mediate two opposing functions, namely pulmonary artery dilation and constriction, in a manner seemingly independent of IP3Rs or calcium influx pathways. Importantly, the work of others had shown that, unlike skeletal and cardiac muscles, smooth muscles might express all three RyR subtypes. If this were the case in our experimental system and cADPR played a role, then 8-bromo-cADPR would surely block one of the opposing RyR-dependent functions identified, or the other, but certainly not both. The latter seemingly implausible scenario was confirmed. How could this be, do cells hold multiple, segregated SR stores that incorporate different RyR subtypes in receipt of spatially segregated signals carried by cADPR? The pharmacological profile of 8-bromo-cADPR action supported not only this, but also indicated that intracellular calcium signals were delivered across intracellular junctions formed by the S/ER. Not just one, at least two. This article retraces the steps along this journey, from the curious pharmacological profile of 8-bromo-cADPR to the discovery of the cell-wide web, a diverse network of cytoplasmic nanocourses demarcated by S/ER nanojunctions, which direct site-specific calcium flux and may thus coordinate the full panoply of cellular processes.

Zinc pyrithione activates K+ channels and hyperpolarizes the membrane of rat pulmonary artery smooth muscle cells

The membrane potential helps determine pulmonary artery smooth muscle cell (PASMC) contraction. The Kv7 channel activators, retigabine and flupirtine, are thought to dilate pulmonary arteries by hyperpolarising PASMC. Zinc pyrithione activates Kv7 channels by a mechanism distinct from retigabine and with different Kv7 subunit selectivity. This study aimed to determine if zinc pyrithione selectively activates Kv7 channels in rat PASMC to evoke pulmonary artery dilation. Zinc pyrithione relaxed pulmonary arteries with half-maximal effect at 4.3μM. At 10μM it activated pronounced voltage-dependent K⁺ current and hyperpolarized PASMCs by around 10mV. Tetraethylammonium ions (TEA, 10mM) and paxilline (1μM) abolished both the current and hyperpolarisation. XE991 (10μM) blocked the hyperpolarization and reduced the current by 30%. Iberiotoxin (50nM) had no effect on the hyperpolarisation, but reduced the current by 40%. The XE991-sensitive current activated with an exponential time course (time constant 17ms), whereas the iberiotoxin-sensitive current followed a bi-exponential time course (time constants 6 and 57ms), suggesting that the drugs blocked different components of the zinc pyrithione-induced current. Zinc pyrithione therefore appears to activate at least two types of K⁺ channel in PASMC; an XE991, TEA and paxilline-sensitive Kv7 channel and a TEA, paxilline and iberiotoxin-sensitive BK_Ca channel. Both could contribute to the relaxing effect of zinc pyrithione on pulmonary artery.

The Role of KV7.3 in Regulating Osteoblast Maturation and Mineralization

KCNQ (KV7) channels are voltage-gated potassium (KV) channels, and the function of KV7 channels in muscles, neurons, and sensory cells is well established. We confirmed that overall blockade of KV channels with tetraethylammonium augmented the mineralization of bone-marrow-derived human mesenchymal stem cells during osteogenic differentiation, and we determined that KV7.3 was expressed in MG-63 and Saos-2 cells at the mRNA and protein levels. In addition, functional KV7 currents were detected in MG-63 cells. Inhibition of KV7.3 by linopirdine or XE991 increased the matrix mineralization during osteoblast differentiation. This was confirmed by alkaline phosphatase, osteocalcin, and osterix in MG-63 cells, whereas the expression of Runx2 showed no significant change. The extracellular glutamate secreted by osteoblasts was also measured to investigate its effect on MG-63 osteoblast differentiation. Blockade of KV7.3 promoted the release of glutamate via the phosphorylation of extracellular signal-regulated kinase 1/2-mediated upregulation of synapsin, and induced the deposition of type 1 collagen. However, activation of KV7.3 by flupirtine did not produce notable changes in matrix mineralization during osteoblast differentiation. These results suggest that KV7.3 could be a novel regulator in osteoblast differentiation.

Hypoxia-dependent reactive oxygen species signaling in the pulmonary circulation: focus on ion channels

SIGNIFICANCE

An acute lack of oxygen in the lung causes hypoxic pulmonary vasoconstriction, which optimizes gas exchange. In contrast, chronic hypoxia triggers a pathological vascular remodeling causing pulmonary hypertension, and ischemia can cause vascular damage culminating in lung edema.

RECENT ADVANCES

Regulation of ion channel expression and gating by cellular redox state is a widely accepted mechanism; however, it remains a matter of debate whether an increase or a decrease in reactive oxygen species (ROS) occurs under hypoxic conditions. Ion channel redox regulation has been described in detail for some ion channels, such as Kv channels or TRPC6. However, in general, information on ion channel redox regulation remains scant.

CRITICAL ISSUES AND FUTURE DIRECTIONS

In addition to the debate of increased versus decreased ROS production during hypoxia, we aim here at describing and deciphering why different oxidants, under different conditions, can cause both activation and inhibition of channel activity. While the upstream pathways affecting channel gating are often well described, we need a better understanding of redox protein modifications to be able to determine the complexity of ion channel redox regulation. Against this background, we summarize the current knowledge on hypoxia-induced ROS-mediated ion channel signaling in the pulmonary circulation.

Redox regulation of ion channels in the pulmonary circulation

SIGNIFICANCE

The pulmonary circulation is a low-pressure, low-resistance, highly compliant vasculature. In contrast to the systemic circulation, it is not primarily regulated by a central nervous control mechanism. The regulation of resting membrane potential due to ion channels is of integral importance in the physiology and pathophysiology of the pulmonary vasculature.

RECENT ADVANCES

Redox-driven ion conductance changes initiated by direct oxidation, nitration, and S-nitrosylation of the cysteine thiols and indirect phosphorylation of the threonine and serine residues directly affect pulmonary vascular tone.

CRITICAL ISSUES

Molecular mechanisms of changes in ion channel conductance, especially the identification of the sites of action, are still not fully elucidated.

FUTURE DIRECTIONS

Further investigation of the interaction between redox status and ion channel gating, especially the physiological significance of S-glutathionylation and S-nitrosylation, could result in a better understanding of the physiological and pathophysiological importance of these mediators in general and the implications of such modifications in cellular functions and related diseases and their importance for targeted treatment strategies.

Functional expression of KCNQ (Kv7) channels in guinea pig bladder smooth muscle and their contribution to spontaneous activity

Background and Purpose

The aim of the study was to determine whether KCNQ channels are functionally expressed in bladder smooth muscle cells (SMC) and to investigate their physiological significance in bladder contractility.

Experimental Approach

KCNQ channels were examined at the genetic, protein, cellular and tissue level in guinea pig bladder smooth muscle using RT-PCR, immunofluorescence, patch-clamp electrophysiology, calcium imaging, detrusor strip myography, and a panel of KCNQ activators and inhibitors.

Key Results

KCNQ subtypes 1–5 are expressed in bladder detrusor smooth muscle. Detrusor strips typically displayed TTX-insensitive myogenic spontaneous contractions that were increased in amplitude by the KCNQ channel inhibitors XE991, linopirdine or chromanol 293B. Contractility was inhibited by the KCNQ channel activators flupirtine or meclofenamic acid (MFA). The frequency of Ca²⁺-oscillations in SMC contained within bladder tissue sheets was increased by XE991. Outward currents in dispersed bladder SMC, recorded under conditions where BK and K_ATP currents were minimal, were significantly reduced by XE991, linopirdine, or chromanol, and enhanced by flupirtine or MFA. XE991 depolarized the cell membrane and could evoke transient depolarizations in quiescent cells. Flupirtine (20 μM) hyperpolarized the cell membrane with a simultaneous cessation of any spontaneous electrical activity.

Conclusions and Implications

These novel findings reveal the role of KCNQ currents in the regulation of the resting membrane potential of detrusor SMC and their important physiological function in the control of spontaneous contractility in the guinea pig bladder.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Driving With No Brakes: Molecular Pathophysiology of Kv7 Potassium Channels

Kv7 potassium channels regulate excitability in neuronal, sensory, and muscular cells. Here, we describe their molecular architecture, physiological roles, and involvement in genetically determined channelopathies highlighting their relevance as targets for pharmacological treatment of several human disorders.

Hypoxia. 4. Hypoxia and ion channel function

The ability to sense and respond to oxygen deprivation is required for survival; thus, understanding the mechanisms by which changes in oxygen are linked to cell viability and function is of great importance. Ion channels play a critical role in regulating cell function in a wide variety of biological processes, including neuronal transmission, control of ventilation, cardiac contractility, and control of vasomotor tone. Since the 1988 discovery of oxygen-sensitive potassium channels in chemoreceptors, the effect of hypoxia on an assortment of ion channels has been studied in an array of cell types. In this review, we describe the effects of both acute and sustained hypoxia (continuous and intermittent) on mammalian ion channels in several tissues, the mode of action, and their contribution to diverse cellular processes.

Contractile and electrophysiological properties of pulmonary artery smooth muscle are not altered in TASK-1 knockout mice

The acid-sensitive, two-pore domain K+ channel, TASK-1, contributes to the background K+ conductance and membrane potential (Em) of rat and human pulmonary artery smooth muscle cells (PASMCs), but its role in regulating tone remains elusive. This study aimed to clarify the role of TASK-1 by determining the functional properties of pulmonary artery (PA) from mice in which the TASK-1 gene was deleted (TASK-1/3 KO), in comparison with wild-type (WT) C57BL/6 controls. Small vessel wire myography was used to measure isometric tension developed by intact PA. Em and currents were recorded from freshly isolated PASMCs using the perforated patch-clamp technique. Reverse transcription-polymerase chain reaction (RT-PCR) was used to estimate K+ channel expression. We could find no difference between PA from WT and TASK-1/3 KO mice. They showed similar constrictor responses to a range of agonists and K+ concentrations, the K+ channel blockers 4-aminopyridine, tetraethylammonium ions and XE991. Treprostinil, proposed to dilate by activating TASK-1, was just as effective in TASK-1/3 KO arteries. Blocking Ca2+ influx with nifedipine (1 μM) or levcromakalim (10 μM) had no effect on resting tone in either strain. The resting Em of PASMCs and its responses to K+ channel blockers were unchanged in TASK-1/3 KO mice as were voltage-activated K+ currents, including the non-inactivating K+ current (IKN) measured at 0 mV. The Em was, however, depolarised in comparison with other species.Mouse IKN was much smaller than in rat and showed no sensitivity to pH. The results imply that TASK-1 does not form a functional channel in mouse PASMCs.

KCNQ potassium channels: new targets for pulmonary vasodilator drugs?

Smooth muscle cells regulate the diameter of pulmonary arteries and the resistance to blood flow in the pulmonary circulation. These cells are normally relaxed to maintain low intrinsic vessel tone, but are contracted in pulmonary arterial hypertension (PAH). Potassium channels in the smooth muscle cell help to maintain low tone by polarising the membrane and preventing Ca(2+) influx through voltage-operated Ca(2+) channels. There is a loss of K(+) channel activity in PAH, so drugs that open K(+) channels are predicted to have a beneficial effect, provided their action can be restricted to the pulmonary circulation. Here we review the myriad of K(+) channels that are expressed in pulmonary arteries and suggest the roles that each might play in regulating pulmonary artery tone. We conclude that members of the KCNQ family of K(+) channels, the most recent K(+) channels to be discovered in pulmonary artery, may be a useful therapeutic target for the treatment of PAH. KCNQ channels appear to be preferentially expressed in pulmonary arteries and drugs that modulate their activity have potent effects on pulmonary artery tone.

Organ culture mimics the effects of hypoxia on membrane potential, K(+) channels and vessel tone in pulmonary artery

BACKGROUND AND PURPOSE

Blood vessel culture is gaining interest for use with transfection-based techniques, but alters the contractile properties of the vessels. The present study tested the effects of culture on the intrinsic tone of rat pulmonary arteries (PAs) and examined the function and expression of K(+) channels regulating the resting membrane potential (E(m)) and tone of pulmonary artery smooth muscle cells (PASMCs).

EXPERIMENTAL APPROACH

Rat intrapulmonary arteries were isolated and cultured under standard and modified conditions. Contractile responses of fresh and cultured PA were compared using vessel myograph. Electrophysiology experiments on isolated PASMCs used the patch-clamp technique. K(+) channel expression was quantified using reverse transcription and real-time PCR.

KEY RESULTS

After 4 days in culture vessels contracted to phenylephrine, but relaxation to carbachol was significantly impaired. Contractile responses to 10 mM KCl, 4-aminopyridine and tetraethylammonium increased, and vessels developed an uncharacteristic relaxation response to Ca(2+)-free solution, nifedipine and levcromakalim. PASMCs from cultured vessels were depolarized and K(+) currents reduced, in association with down-regulation of K(v)1.5, K(v)2.1 and TWIK-related acid-sensitive K(+) channel-1 mRNA. These changes were partially reversed by increased oxygenation of the culture medium or removing the endothelium before culture.

CONCLUSIONS AND IMPLICATIONS

Culture of PA for 3-4 days induced loss of functional K(+) channels, depolarization of PASMCs, Ca(2+) influx, intrinsic tone and spontaneous constrictions, similar to the effects of chronic hypoxia. This limits the use of cultured vessels for studying excitation-contraction coupling, although oxygenating the culture medium and removing the endothelium can help to retain normal smooth muscle function.

Treatment with the Kv7 potassium channel activator flupirtine is beneficial in two independent mouse models of pulmonary hypertension

BACKGROUND AND PURPOSE

Voltage-gated potassium (K(v)) channels contribute to resting membrane potential in pulmonary artery smooth muscle cells and are down regulated in patients with pulmonary arterial hypertension (PAH) and a contribution from K(v)7 channels has been recently proposed. We investigated the effect of the K(v)7 channel activator, flupirtine, on PAH in two independent mouse models: PAH induced by hypoxia and spontaneous PAH in mice over-expressing the 5-HT transporter (SERT(+) mice).

EXPERIMENTAL APPROACH

Right ventricular pressure was assessed in vivo in mice chronically treated with flupirtine (30 mg.kg(-1).day(-1)). In separate in vitro experiments, pulmonary arteries from untreated mice were mounted in a wire myograph. Relaxations to acute administration of flupirtine and contractions to K(v) channel blocking drugs, including the K(v)7 channel blocker linopirdine, were measured.

KEY RESULTS

In wild-type (WT) mice, hypoxia increased right ventricular pressure, pulmonary vascular remodelling and right ventricular hypertrophy. These effects were attenuated by flupirtine, which also attenuated these indices of PAH in SERT(+) mice. In the in vitro experiments, flupirtine induced a potent relaxant response in arteries from untreated WT and SERT(+) mice. The relaxation was fully reversed by linopirdine, which potently contracted mouse pulmonary arteries while other K(v) channel blockers did not.

CONCLUSIONS AND IMPLICATIONS

Flupirtine significantly attenuated development of chronic hypoxia-induced PAH in mice and reversed established PAH in SERT(+) mice, apparently via K(v)7 channel activation. These results provide the first direct evidence that drugs activating K(v)7 channels may be of benefit in the treatment of PAH with different aetiologies.

Mechanisms of U46619-induced contraction of rat pulmonary arteries in the presence and absence of the endothelium

BACKGROUND AND PURPOSE

Thromboxane A(2) and endothelial dysfunction are implicated in the development of pulmonary hypertension. The receptor-transduction pathway for U46619 (9,11-dideoxy-9 alpha, 11 alpha-methanoepoxy prostaglandin F(2 alpha))-induced contraction was examined in endothelium-intact (E+) and denuded (E-) rat pulmonary artery rings.

EXPERIMENTAL APPROACH

Artery rings were mounted on a wire myograph under a tension of 7-7.5 mN at 37 degrees C and gassed with 95% O(2)/5% CO(2). Isometric recording was made by using Powerlab data collection and Chart 5 software.

KEY RESULTS

Both E+ and E- contractile responses were sensitive to Rho-kinase inhibition and the chloride channel blocker NPPB [5-nitro-2-(3-phenylpropylamino)benzoic acid]. The E+ response was sensitive to the store-operated calcium channel blockers SKF-96365 {1-[B-[3-(4-methoxyphenyl)propoxy]-4-methoxy-phenethyl]-1H-imidazole hydrochloride} and 2-APB (2-amino ethoxy diphenylborate) (75-100 micromol x L(-1)). The E- response was sensitive to 2-APB (10-30 micromol x L(-1)), a putative IP(3) receptor antagonist, and the calcium and chloride channel blockers nifedipine, DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) and niflumic acid but was insensitive to SKF-96365. Inhibiting K(V) with 4-AP in E+ rings exposed a contraction sensitive to nifedipine, DIDS and niflumic acid, whereas inhibiting BK(Ca) exposed a contraction sensitive to mibefradil, DIDS and niflumic acid. This indicates that removal of the endothelium allows the TP receptor to inhibit K(V), which may involve coupling to phospholipase C, because inhibition of phospholipase C with U73122 (1-[6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-y]amino]hexyl]- 1H-pyrrole-2,5-dione) switched the E- pathway to the E+ pathway.

CONCLUSIONS AND IMPLICATIONS

The results from this study indicate that distinct transduction pathways can be employed by the TP receptor to produce contraction and that the endothelium is able to influence the coupling of the TP receptor.

KCNQ modulators reveal a key role for KCNQ potassium channels in regulating the tone of rat pulmonary artery smooth muscle

Potassium channels are central to the regulation of pulmonary vascular tone. The smooth muscle cells of pulmonary artery display a background K(+) conductance with biophysical properties resembling those of KCNQ (K(V)7) potassium channels. Therefore, we investigated the expression and functional role of KCNQ channels in pulmonary artery. The effects of selective KCNQ channel modulators were investigated on K(+) current and membrane potential in isolated pulmonary artery smooth muscle cells (PASMCs), on the tension developed by intact pulmonary arteries, and on pulmonary arterial pressure in isolated perfused lungs and in vivo. The KCNQ channel blockers, linopirdine and XE991 [10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone], inhibited the noninactivating background K(+) conductance in PASMCs and caused depolarization, vasoconstriction, and raised pulmonary arterial pressure without constricting several systemic arteries or raising systemic pressure. The KCNQ channel openers, retigabine and flupirtine, had the opposite effects. PASMCs were found to express KCNQ4 mRNA, at higher levels than mesenteric artery, along with smaller amounts of KCNQ1 and 5. It is concluded that KCNQ channels, most probably KCNQ4, make an important contribution to the regulation of pulmonary vascular tone, with a greater contribution in pulmonary compared with systemic vessels. The pulmonary vasoconstrictor effect of KCNQ blockers is a potentially serious side effect, but the pulmonary vasodilator effect of the openers may be useful in the treatment of pulmonary hypertension.

Bone morphogenetic protein-2 upregulates expression and function of voltage-gated K+ channels in human pulmonary artery smooth muscle cells

Activity of voltage-gated K(+) (K(V)) channels in pulmonary artery smooth muscle cells (PASMC) plays an important role in control of apoptosis and proliferation in addition to regulating membrane potential and pulmonary vascular tone. Bone morphogenetic proteins (BMPs) inhibit proliferation and induce apoptosis in normal human PASMC, whereas dysfunctional BMP signaling and downregulated K(V) channels are involved in pulmonary vascular medial hypertrophy associated with pulmonary hypertension. This study evaluated the effect of BMP-2 on K(V) channel function and expression in normal human PASMC. BMP-2 (100 nM for 18-24 h) significantly (>2-fold) upregulated mRNA expression of KCNA5, KCNA7, KCNA10, KCNC3, KCNC4, KCNF1, KCNG3, KCNS1, and KCNS3 but downregulated (at least 2-fold) KCNAB1, KCNA2, KCNG2, and KCNV2. The most dramatic change was the >10-fold downregulation of KCNG2 and KCNV2, two electrically silent gamma-subunits that form heterotetramers with functional K(V) channel alpha-subunits (e.g., KCNB1-2). Furthermore, the amplitude and current density of whole cell K(V) currents were significantly increased in PASMC treated with BMP-2. It has been demonstrated that K(+) currents generated by KCNB1 and KCNG1 (or KCNG2) or KCNB1 and KCNV2 heterotetramers are smaller than those generated by KCNB1 homotetramers, indicating that KCNG2 and KCNV2 (2 subunits that were markedly downregulated by BMP-2) are inhibitors of functional K(V) channels. These results suggest that BMP-2 divergently regulates mRNA expression of various K(V) channel alpha-, beta-, and gamma-subunits and significantly increases whole cell K(V) currents in human PASMC. Finally, we present evidence that attenuation of c-Myc expression by BMP-2 may be involved in BMP-2-mediated increase in K(V) channel activity and regulation of K(V) channel expression. The increased K(V) channel activity may be involved in the proapoptotic and/or antiproliferative effects of BMP-2 on PASMC.

Pulmonary vasoconstrictor action of KCNQ potassium channel blockers

BACKGROUND

KCNQ channels have been widely studied in the nervous system, heart and inner ear, where they have important physiological functions. Recent reports indicate that KCNQ channels may also be expressed in portal vein where they are suggested to influence spontaneous contractile activity. The biophysical properties of K+ currents mediated by KCNQ channels resemble a current underlying the resting K+ conductance and resting potential of pulmonary artery smooth muscle cells. We therefore investigated a possible role of KCNQ channels in regulating the function of pulmonary arteries by determining the ability of the selective KCNQ channel blockers, linopirdine and XE991, to promote pulmonary vasoconstriction.

METHODS

The tension developed by rat and mouse intrapulmonary or mesenteric arteries was measured using small vessel myography. Contractile responses to linopirdine and XE991 were measured in intact and endothelium denuded vessels. Experiments were also carried out under conditions that prevent the contractile effects of nerve released noradrenaline or ATP, or block various Ca2+ influx pathways, in order to investigate the mechanisms underlying contraction.

RESULTS

Linopirdine and XE991 both contracted rat and mouse pulmonary arteries but had little effect on mesenteric arteries. In each case the maximum contraction was almost as large as the response to 50 mM K+. Linopirdine had an EC50 of around 1 microM and XE991 was almost 10-fold more potent. Neither removal of the endothelium nor exposure to phentolamine or alpha,beta-methylene ATP, to block alpha1-adrenoceptors or P2X receptors, respectively, affected the contraction. Contraction was abolished in Ca2+-free solution and in the presence of 1 microM nifedipine or 10 microM levcromakalim.

CONCLUSION

The KCNQ channel blockers are potent and powerful constrictors of pulmonary arteries. This action may be selective for the pulmonary circulation as mesenteric arteries showed little response. The results imply that the drugs act directly on smooth muscle cells and contraction requires voltage-dependent Ca2+ influx. It is concluded that the drugs probably act by blocking KCNQ channels in pulmonary artery myocytes, leading to membrane depolarization and Ca2+ influx through L-type Ca2+ channels. This implies a functional role for KCNQ channels in regulating the resting membrane potential of pulmonary artery myocytes.

Electrophysiological and functional effects of the KCNQ channel blocker XE991 on murine portal vein smooth muscle cells

The effect of the KCNQ channel blockers XE991, chromanol 293B and linopirdine, was studied on voltage-dependent K+ currents in smooth muscle cells dissociated freshly from mouse portal vein (mPV) and isometric tension recordings from whole mPV. Voltage clamp experiments showed XE991 inhibited an outward current in a concentration-dependent manner with an IC50 of 5.8 microM. Block was voltage independent. Chromanol 293B and linopirdine also blocked the voltage-dependent K+ current but were less potent than XE991. At least two components--a linear (I(linear)) and an outward relaxation (I(out))--contributed to the XE991-sensitive conductance. XE991-sensitive currents were sustained at all test potentials and XE991 inhibited the enhanced holding current at -60 mV produced by bathing cells in an external solution containing 36 mM KCl. Current clamp experiments in the perforated-patch configuration showed XE991 and linopirdine depolarised the resting membrane potential and augmented the evoked response in a concentration-dependent manner. In functional experiments the spontaneous contractile activity of the mPV was increased significantly by XE991 and linopirdine. The stimulatory effect of XE991 was not affected by the presence of 4-AP, glibenclamide nor paxilline. These data provide evidence for an important role for KCNQ channels in governing cellular excitability in mPV smooth muscle cells.

Ion channels in smooth muscle: regulators of intracellular calcium and contractility

Smooth muscle (SM) is essential to all aspects of human physiology and, therefore, key to the maintenance of life. Ion channels expressed within SM cells regulate the membrane potential, intracellular Ca2+ concentration, and contractility of SM. Excitatory ion channels function to depolarize the membrane potential. These include nonselective cation channels that allow Na+ and Ca2+ to permeate into SM cells. The nonselective cation channel family includes tonically active channels (Icat), as well as channels activated by agonists, pressure-stretch, and intracellular Ca2+ store depletion. Cl--selective channels, activated by intracellular Ca2+ or stretch, also mediate SM depolarization. Plasma membrane depolarization in SM activates voltage-dependent Ca2+ channels that demonstrate a high Ca2+ selectivity and provide influx of contractile Ca2+. Ca2+ is also released from SM intracellular Ca2+ stores of the sarcoplasmic reticulum (SR) through ryanodine and inositol trisphosphate receptor Ca2+ channels. This is part of a negative feedback mechanism limiting contraction that occurs by the Ca2+-dependent activation of large-conductance K+ channels, which hyper polarize the plasma membrane. Unlike the well-defined contractile role of SR-released Ca2+ in skeletal and cardiac muscle, the literature suggests that in SM Ca2+ released from the SR functions to limit contractility. Depolarization-activated K+ chan nels, ATP-sensitive K+ channels, and inward rectifier K+ channels also hyperpolarize SM, favouring relaxation. The expression pattern, density, and biophysical properties of ion channels vary among SM types and are key determinants of electrical activity, contractility, and SM function.

Anti-proliferating effect of iptakalim, a novel KATP channel opener, in cultured rabbit pulmonary arterial smooth muscle cells

ATP-sensitive potassium (K(ATP)) channels of pulmonary arterial smooth muscle cells (SMCs) have been implicated in pulmonary hypertension. Iptakalim, designed and synthesized by ourselves, is a newly selective K(ATP) channel opener. Here, we explored the effects of iptakalim on the rise of cytoplasmic free Ca(2+) concentration ([Ca(2+)](cyt)) induced by endothelin-1 (ET-1) and on the proliferation of cultured rabbit pulmonary arterial SMCs. The results showed that iptakalim inhibited the [Ca(2+)](cyt) increase. The enhanced [(3)H]thymidine incorporation was inhibited and the transition of cells from static phase (G(0)/G(1)) to DNA synthesis (S) and mitotic phase (G(2)/M) was held back by iptakalim in a concentration-dependent manner. Glyburide abolished the inhibitory effect of iptakalim. In conclusion, we have shown that iptakalim had an inhibitory effect on [Ca(2+)](cyt) increase and the proliferation of pulmonary arterial SMCs induced by endothelin-1 through activation of K(ATP) channels. These findings suggest that iptakalim might be a promising candidate for the treatment of pulmonary arterial remodeling in pulmonary hypertension.

Effects of iptakalim hydrochloride, a novel KATP channel opener, on pulmonary vascular remodeling in hypoxic rats

To investigate whether pulmonary artery remodeling could be prevented or not in hypoxic pulmonary hypertensive rats by treatment, the effects of iptakalim hydrochloride, a novel KATPCO, were evaluated. Iptakalim hydrochloride was orally administered at the doses of either 1.5 mg/kg/day or 0.75 mg/kg/day before their 4-week exposure to hypoxia (10% oxygen). It was demonstrated that iptakalim hydrochloride could reverse all pathological indices of pulmonary arterial remodeling and significantly reduce right ventricular hypertrophy in hypoxic rats. The reversal of hypoxic indices was dose-dependent, in which the higher dose of iptakalim hydrochloride reversed pathological indices more effectively than the lower dose did. This was further confirmed electrophysiologically using whole cell patch-clamp technique, which revealed that the outward potassium currents could be enhanced by iptakalim hydrochloride, and the decrease of K+ current density and increase of membrane capacitance could be reversed by chronic iptakalim hydrochloride treatment. These findings implied that iptakalim hydrochloride could play its role through activating plasmalemmal K+ channels of pulmonary arterial SMCs. The results indicated that iptakalim hydrochloride had anti-remodeling properties of pulmonary artery in hypoxic pulmonary hypertensive rats. It is therefore suggested that KATPCOs might be promising in the treatment of patients with hypoxic, and even possibly other forms of, pulmonary hypertension.

Two-pore domain K channel, TASK-1, in pulmonary artery smooth muscle cells

Pulmonary vascular tone is strongly influenced by the resting membrane potential of smooth muscle cells, depolarization promoting Ca2+ influx, and contraction. The resting potential is determined largely by the activity of K+-selective ion channels, the molecular nature of which has been debated for some time. In this study, we provide strong evidence that the two-pore domain K+ channel, TASK-1, mediates a noninactivating, background K+ current (IKN), which sets the resting membrane potential in rabbit pulmonary artery smooth muscle cells (PASMCs). TASK-1 mRNA was found to be present in PASMCs, and the membranes of PASMCs contained TASK-1 protein. Both IKN and the resting potential were found to be exquisitely sensitive to extracellular pH, acidosis inhibiting the current and causing depolarization. Moreover, IKN and the resting potential were enhanced by halothane (1 mmol/L), inhibited by Zn2+ (100 to 200 micromol/L) and anandamide (10 micromol/L), but insensitive to cytoplasmic Ca2+. These properties are all diagnostic of TASK-1 channels and add to previously identified features of IKN that are shared with TASK-1, such as inhibition by hypoxia, low sensitivity to 4-aminopyridine and quinine and insensitivity to tetraethylammonium ions. It is therefore concluded that TASK-1 channels are major contributors to the resting potential in pulmonary artery smooth muscle. They are likely to play an important role in mediating pulmonary vascular responses to changes in extracellular pH, and they could be responsible for the modulatory effects of pH on hypoxic pulmonary vasoconstriction.

Vasodilation by the calcium-mobilizing messenger cyclic ADP-ribose

In artery smooth muscle, adenylyl cyclase-coupled receptors such as beta-adrenoceptors evoke Ca(2+) signals, which open Ca(2+)-activated potassium (BK(Ca)) channels in the plasma membrane. Thus, blood pressure may be lowered, in part, through vasodilation due to membrane hyperpolarization. The Ca(2+) signal is evoked via ryanodine receptors (RyRs) in sarcoplasmic reticulum proximal to the plasma membrane. We show here that cyclic adenosine diphosphate-ribose (cADPR), by activating RyRs, mediates, in part, hyperpolarization and vasodilation by beta-adrenoceptors. Thus, intracellular dialysis of cADPR increased the cytoplasmic Ca(2+) concentration proximal to the plasma membrane in isolated arterial smooth muscle cells and induced a concomitant membrane hyperpolarization. Smooth muscle hyperpolarization mediated by cADPR, by beta-adrenoceptors, and by cAMP, respectively, was abolished by chelating intracellular Ca(2+) and by blocking RyRs, cADPR, and BK(Ca) channels with ryanodine, 8-amino-cADPR, and iberiotoxin, respectively. The cAMP-dependent protein kinase A antagonist N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide hydrochloride (H89) blocked hyperpolarization by isoprenaline and cAMP, respectively, but not hyperpolarization by cADPR. Thus, cADPR acts as a downstream element in this signaling cascade. Importantly, antagonists of cADPR and BK(Ca) channels, respectively, inhibited beta-adrenoreceptor-induced artery dilation. We conclude, therefore, that relaxation of arterial smooth muscle by adenylyl cyclase-coupled receptors results, in part, from a cAMP-dependent and protein kinase A-dependent increase in cADPR synthesis, and subsequent activation of sarcoplasmic reticulum Ca(2+) release via RyRs, which leads to activation of BK(Ca) channels and membrane hyperpolarization.

A-type potassium currents in smooth muscle

A-type currents are voltage-gated, calcium-independent potassium (Kv) currents that undergo rapid activation and inactivation. Commonly associated with neuronal and cardiac cell-types, A-type currents have also been identified and characterized in vascular, genitourinary, and gastrointestinal smooth muscle cells. This review examines the molecular identity, biophysical properties, pharmacology, regulation, and physiological function of smooth muscle A-type currents. In general, this review is intended to facilitate the comparison of A-type currents present in different smooth muscles by providing a comprehensive report of the literature to date. This approach should also aid in the identification of areas of research requiring further attention.

Nicotinic acid adenine dinucleotide phosphate mediates Ca2+ signals and contraction in arterial smooth muscle via a two-pool mechanism

Previous studies of arterial smooth muscle have shown that inositol 1,4,5-trisphosphate (IP3) and cyclic ADP-ribose mobilize Ca2+ from the sarcoplasmic reticulum. In contrast, little is known about Ca2+ mobilization by nicotinic acid adenine dinucleotide phosphate, a pyridine nucleotide derived from beta-NADP+. We show here that intracellular dialysis of nicotinic acid adenine dinucleotide phosphate (NAADP) induces spatially restricted "bursts" of Ca2+ release that initiate a global Ca2+ wave and contraction in pulmonary artery smooth muscle cells. Depletion of sarcoplasmic reticulum Ca2+ stores with thapsigargin and inhibition of ryanodine receptors with ryanodine, respectively, block the global Ca2+ waves by NAADP. Under these conditions, however, localized Ca2+ bursts are still observed. In contrast, xestospongin C, an IP3 receptor antagonist, had no effect on Ca2+ signals by NAADP. We propose that NAADP mobilizes Ca2+ via a 2-pool mechanism, and that initial Ca2+ bursts are amplified by subsequent sarcoplasmic reticulum Ca2+ release via ryanodine receptors but not via IP3 receptors.

Chronic carbon monoxide enhanced IbTx-sensitive currents in rat resistance pulmonary artery smooth muscle cells

Exogenous carbon monoxide (CO) can induce pulmonary vasodilation by acting directly on pulmonary artery (PA) smooth muscle cells. We investigated the contribution of K+ channels to the regulation of resistance PA resting membrane potential on control (PAC) rats and rats exposed to CO for 3 wk at 530 parts/million, labeled as PACO rats. Whole cell patch-clamp experiments revealed that the resting membrane potential of PACO cells was more negative than that of PAC cells. This was associated with a decrease of membrane resistance in PACO cells. Additional analysis showed that outward current density in PACO cells was higher (50% at +60 mV) than in PAC cells. This was linked to an increase of iberiotoxin (IbTx)-sensitive current. Chronic CO hyperpolarized membrane of pressurized PA from -46.9 +/- 1.2 to -56.4 +/- 2.6 mV. Additionally, IbTx significantly depolarized membrane of smooth muscle cells from PACO arteries but not from PAC arteries. The present study provides initial evidence of an increase of Ca2+-activated K+ current in smooth muscle cells from PA of rats exposed to chronic CO.

Potassium channels underlying the resting potential of pulmonary artery smooth muscle cells

1. The molecular identity of the K channels giving rise to the negative membrane potential of pulmonary artery smooth muscle cells has yet to be determined. 2. To date, most studies have focused on voltage-gated, delayed rectifier channels and their roles in mediating hypoxia-induced membrane depolarization. There is, however, strong evidence that an outwardly rectifying K+ conductance distinct from the classical delayed rectifier is involved. 3. Growing evidence that TASK-like channels can sense hypoxia and are present in pulmonary artery smooth muscle cells suggests that they may be responsible for the resting K+ conductance and resting potential. 4. The present review considers the evidence that particular K channels maintain the resting membrane potential of pulmonary artery smooth muscle cells and mediate the depolarizing response to hypoxia.

Heterogeneous intrinsic firing properties of vertebrate retinal ganglion cells

Retinal ganglion cells (RGCs) use their characteristic firing patterns to encode various aspects of visual information and carry them to the brain. It has been thought that the firing pattern of an RGC's light response is determined primarily by the time course and spatiotemporal interaction of the synaptic inputs. However, it is unclear whether there is a difference in intrinsic firing properties among RGCs that could contribute to the cell-to-cell distinction of the light response firing pattern. We investigated the intrinsic firing properties of isolated goldfish RGCs, minimizing cytoplasmic disturbance with a perforated-patch, whole-cell recording technique. In response to a 1-s depolarizing current step, the majority of the examined RGCs (n = 84) displayed sustained firing that lasted over 800 ms (n = 24; tonic RGCs) or transient firing accommodated within 200 ms of the step onset (n = 47; phasic RGCs). Tonic and phasic RGCs also differed in their firing frequency-current intensity dynamics. There was a significant difference in the soma sizes of phasic and tonic RGCs, indicating that some parts of these groups originate from distinct morphological subtypes. In the presence of extracellular Ba(2+) (1 mM), phasic RGCs displayed sustained firing and firing frequency-current intensity dynamics similar to those of tonic RGCs. Thus a Ba(2+)-sensitive ion current (I(Ba-s)) underlies the firing characteristics of phasic RGCs. Under voltage-clamp conditions, I(Ba-s) was identified as a low-threshold, noninactivating voltage-dependent K(+) current. Because of its slow kinetics (time constant of activation, approximately 100 ms), I(Ba-s) may confer a gradually increasing hyperpolarizing driving force during maintained excitatory stimulus, which eventually would result in firing accommodation. These findings suggest that RGCs have heterogeneous intrinsic firing properties that could aid synaptic inputs in shaping light responses.

Role of K+ channels in pulmonary hypertension

Pulmonary hypertension is a hemodynamic abnormality that is common to a variety of conditions. In obliterative pulmonary hypertension, vascular remodeling leads to an obliterative process involving the small muscular pulmonary arteries, thereby increasing pulmonary vascular resistance (PVR) and the pulmonary artery pressure (PAP). This process can be triggered by a defect in the function of K+ channels or by alveolar hypoxia. In fact, hypoxia has been shown to selectively inhibit the function and expression of voltage-gated K+ (KV) channels in pulmonary arterial smooth muscle cells (SMCs). K+ channel dysfunction, therefore, plays an important role in the development of pulmonary hypertension. Activity of K+ channels regulates the membrane potential (Em) of SMCs, which in turn regulates cytoplasmic free Ca2+ concentration ([Ca2+]cyt). Depolarization of the Em leads to an elevated [Ca2+]cyt by opening voltage-dependent Ca2+ channels. Elevated [Ca2+]cyt is implicated in stimulating vascular SMC proliferation and inducing vasomotor tone, and hence, vasoconstriction. Vasoconstriction causes elevation of intravascular pressure and elastic stretch of the SMCs, both of which have been shown to play a role in pulmonary arterial cellular growth and synthetic activity, creating a vicious cycle of cellular hypertrophy, proliferation, and vascular remodeling. Dysfunction of K+ channels has also been linked to decreased apoptosis in pulmonary arterial SMCs, a condition that contributes further to the medial hypertrophy of the arterial walls and vascular remodeling. The goal of this article is to review the current understanding of the function of K+ channels and their contribution to the pathophysiology and cellular mechanisms involved in the development of pulmonary hypertension.

Molecular basis of hypoxia-induced pulmonary vasoconstriction: role of voltage-gated K+ channels

The hypoxia-induced membrane depolarization and subsequent constriction of small resistance pulmonary arteries occurs, in part, via inhibition of vascular smooth muscle cell voltage-gated K+ (KV) channels open at the resting membrane potential. Pulmonary arterial smooth muscle cell KV channel expression, antibody-based dissection of the pulmonary arterial smooth muscle cell K+ current, and the O2 sensitivity of cloned KV channels expressed in heterologous expression systems have all been examined to identify the molecular components of the pulmonary arterial O2-sensitive KV current. Likely components include Kv2.1/Kv9.3 and Kv1.2/Kv1.5 heteromeric channels and the Kv3.1b alpha-subunit. Although the mechanism of KV channel inhibition by hypoxia is unknown, it appears that KV alpha-subunits do not sense O2 directly. Rather, they are most likely inhibited through interaction with an unidentified O2 sensor and/or beta-subunit. This review summarizes the role of KV channels in hypoxic pulmonary vasoconstriction, the recent progress toward the identification of KV channel subunits involved in this response, and the possible mechanisms of KV channel regulation by hypoxia.

Inhibition of K(+) channel activity in human pulmonary artery smooth muscle cells by serum from patients with pulmonary hypertension secondary to congenital heart disease

Activity of K(+) channels regulates cytosolic free Ca(2+) concentration by controlling membrane potential. A rise in cytosolic free Ca(2+) concentration in pulmonary artery smooth muscle cells (PASMC) triggers pulmonary vasoconstriction and stimulates PASMC proliferation. Whether serum from children with pulmonary hypertension (PH) secondary to congenital cardiopulmonary diseases contains a factor(s) that inhibits K(+) channel function in PASMC was investigated using patch clamp techniques. PASMC isolated from normal subjects were cultured in media containing 5% serum from normotensive (NPH) or PH patients. Cell growth rate and the currents through voltage-gated K(+) channels were determined and compared between the cells treated with serum from NPH and PH patients. In the absence of growth factors, incubation of PASMC in media containing NPH serum for 48 h increased cell numbers by 2.5-fold, whereas incubation of the cells in media containing PH serum increased cell numbers by 4.5-fold (p < 0.001). Amplitude of whole-cell voltage-gated K(+) currents in NPH serum-treated cells (1119 +/- 222 pA at +80 mV, n = 43) was 3.5-fold greater than in PH serum-treated cells (323 +/- 34 pA, n = 43, p < 0.001). Consistently, membrane potential was much more depolarized in PASMC treated with PH serum (-28 +/- 2 mV, n = 29) than cells treated with NPH-serum (-47 +/- 2 mV, n = 28; p < 0.001). These results suggest that a circulating mitogenic agonist, which induces membrane depolarization by inhibiting voltage-gated K(+) channel activity in PASMC, may be produced or up-regulated in pediatric patients with secondary PH.

Oxygen dilation in fetal pulmonary arterioles: role of K(+) channels

BACKGROUND

Oxygen is a potent stimulus for pulmonary vasodilation. Potassium channels have been implicated as both sensors and effectors for oxygen-induced changes in pulmonary vascular tone. We have examined the effect of potassium channel blockers on oxygen-induced vasodilation in isolated pulmonary arterioles from fetal rats at term.

MATERIALS AND METHODS

Third generation pulmonary arterioles were isolated from fetal rats on Day 22 of gestation, cannulated, pressurized at constant distending pressures, and preconstricted by suffusion with a salt solution bubbled with a "hypoxic gas" mixture (pO(2) <or=50 mm Hg). Oxygen-induced vasodilation was measured as percentage reversal of the "hypoxic" vasoconstriction after 30 min of suffusion with "normoxic" solution (pO(2) 90-145 mm Hg). Responses were recorded in the absence of blockers (controls) or in the presence of a voltage-gated K(+) channel (K(v)) blocker, 4-aminopyridine; an ATP-sensitive K(+) channel (K(ATP)) blocker, glibenclamide; a Ca(2+)-activated K(+) channel (K(Ca)) blocker, charybdotoxin; or a nonspecific K(+) channel blocker, tetraethylammonium.

RESULTS

In control arterioles, normoxic suffusion for 30 min reversed hypoxic preconstriction by 83 +/- 19%. 4-aminopyridine significantly attenuated (44 +/- 9%), and glibenclamide and charybdotoxin had no effect (80 +/- 16 and 79 +/- 20%) on the magnitude of normoxic vasodilation.

CONCLUSIONS

Our results are consistent with a contribution of K(v) channels, but not K(ATP) or K(Ca) channels, to oxygen-induced vasodilation in third generation pulmonary arterioles from term fetal rats.

Adp-ribosyl cyclase and cyclic ADP-ribose hydrolase act as a redox sensor. a primary role for cyclic ADP-ribose in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction is unique to pulmonary arteries and serves to match lung perfusion to ventilation. However, in disease states this process can promote hypoxic pulmonary hypertension. Hypoxic pulmonary vasoconstriction is associated with increased NADH levels in pulmonary artery smooth muscle and with intracellular Ca(2+) release from ryanodine-sensitive stores. Because cyclic ADP-ribose (cADPR) regulates ryanodine receptors and is synthesized from beta-NAD(+), we investigated the regulation by beta-NADH of cADPR synthesis and metabolism and the role of cADPR in hypoxic pulmonary vasoconstriction. Significantly higher rates of cADPR synthesis occurred in smooth muscle homogenates of pulmonary arteries, compared with homogenates of systemic arteries. When the beta-NAD(+):beta-NADH ratio was reduced, the net amount of cADPR accumulated increased. This was due, at least in part, to the inhibition of cADPR hydrolase by beta-NADH. Furthermore, hypoxia induced a 10-fold increase in cADPR levels in pulmonary artery smooth muscle, and a membrane-permeant cADPR antagonist, 8-bromo-cADPR, abolished hypoxic pulmonary vasoconstriction in pulmonary artery rings. We propose that the cellular redox state may be coupled via an increase in beta-NADH levels to enhanced cADPR synthesis, activation of ryanodine receptors, and sarcoplasmic reticulum Ca(2+) release. This redox-sensing pathway may offer new therapeutic targets for hypoxic pulmonary hypertension.

Chronic hypoxia decreases K(V) channel expression and function in pulmonary artery myocytes

Activity of voltage-gated K+ (KV) channels regulates membrane potential (E(m)) and cytosolic free Ca2+ concentration ([Ca2+](cyt)). A rise in ([Ca2+](cyt))in pulmonary artery (PA) smooth muscle cells (SMCs) triggers pulmonary vasoconstriction and stimulates PASMC proliferation. Chronic hypoxia (PO(2) 30-35 mmHg for 60-72 h) decreased mRNA expression of KV channel alpha-subunits (Kv1.1, Kv1.5, Kv2.1, Kv4.3, and Kv9.3) in PASMCs but not in mesenteric artery (MA) SMCs. Consistently, chronic hypoxia attenuated protein expression of Kv1.1, Kv1.5, and Kv2.1; reduced KV current [I(KV)]; caused E(m) depolarization; and increased ([Ca2+](cyt)) in PASMCs but negligibly affected KV channel expression, increased I(KV), and induced hyperpolarization in MASMCs. These results demonstrate that chronic hypoxia selectively downregulates KV channel expression, reduces I(KV), and induces E(m) depolarization in PASMCs. The subsequent rise in ([Ca2+](cyt)) plays a critical role in the development of pulmonary vasoconstriction and medial hypertrophy. The divergent effects of hypoxia on KV channel alpha-subunit mRNA expression in PASMCs and MASMCs may result from different mechanisms involved in the regulation of KV channel gene expression.

Sustained membrane depolarization and pulmonary artery smooth muscle cell proliferation

Pulmonary vasoconstriction and vascular medial hypertrophy greatly contribute to the elevated pulmonary vascular resistance in patients with pulmonary hypertension. A rise in cytosolic free Ca(2+) ([Ca(2+)](cyt)) in pulmonary artery smooth muscle cells (PASMC) triggers vasoconstriction and stimulates cell growth. Membrane potential (E(m)) regulates [Ca(2+)](cyt) by governing Ca(2+) influx through voltage-dependent Ca(2+) channels. Thus intracellular Ca(2+) may serve as a shared signal transduction element that leads to pulmonary vasoconstriction and vascular remodeling. In PASMC, activity of voltage-gated K(+) (Kv) channels regulates resting E(m). In this study, we investigated whether changes of Kv currents [I(K(V))], E(m), and [Ca(2+)](cyt) affect cell growth by comparing these parameters in proliferating and growth-arrested PASMC. Serum deprivation induced growth arrest of PASMC, whereas chelation of extracellular Ca(2+) abolished PASMC growth. Resting [Ca(2+)](cyt) was significantly higher, and resting E(m) was more depolarized, in proliferating PASMC than in growth-arrested cells. Consistently, whole cell I(K(V)) was significantly attenuated in PASMC during proliferation. Furthermore, E(m) depolarization significantly increased resting [Ca(2+)](cyt) and augmented agonist-mediated rises in [Ca(2+)](cyt) in the absence of extracellular Ca(2+). These results demonstrate that reduced I(K(V)), depolarized E(m), and elevated [Ca(2+)](cyt) may play a critical role in stimulating PASMC proliferation. Pulmonary vascular medial hypertrophy in patients with pulmonary hypertension may be partly caused by a membrane depolarization-mediated increase in [Ca(2+)](cyt) in PASMC.

Molecular identification of O2 sensors and O2-sensitive potassium channels in the pulmonary circulation

Small, muscular pulmonary arteries (PAs) constrict within seconds of the onset of alveolar hypoxia, diverting blood flow to better-ventilated lobes, thereby matching ventilation to perfusion and optimizing systemic PO2. This hypoxic pulmonary vasoconstriction (HPV) is enhanced by endothelial derived vasoconstrictors, such as endothelin, and inhibited by endothelial derived nitric oxide. However, the essence of the response is intrinsic to PA smooth muscle cells in resistance arteries (PASMCs). HPV is initiated by inhibition of the Kv channels in PASMCs which set the membrane potential (EM). It is currently uncertain whether this reflects an initial inhibitory effect of hypoxia on the K+ channels or an initial release of intracellular Ca2+, which then inhibits K+ channels. In either scenario, the resulting depolarization activates L-type, voltage gated Ca2+ channels, which raises cytosolic calcium levels [Ca2+]i and causes vasoconstriction. Nine families of Kv channels are recognized from cloning studies (Kv1-Kv9), each with subtypes (i.e. Kv1.1-1.6). The contribution of an individual Kv channel to the whole-cell current (IK) is difficult to determine pharmacologically because Kv channel inhibitors are nonspecific. Furthermore, the PASMC's IK is an ensemble, reflecting activity of several channels. The K+ channels which set EM, and inhibition of which initiates HPV, conduct an outward current which is slowly inactivating, and which is blocked by the Kv inhibitor 4-aminopyridine (4-AP) but not by inhibitors of Ca(2+)- or ATP-sensitive K+ channels. Using anti-Kv antibodies to immunolocalize and inhibit Kv channels, we showed that the PASMC contains numerous types of Kv channels from the Kv1 and Kv2 families., Furthermore Kv1.5 and Kv2.1 may be important in determining the EM and play a role as effectors of HPV in PASMCs. While the Kv channels in PASMCs are the "effectors" of HPV, it is uncertain whether they are intrinsically O2-sensitive or are under the control of an "O2 sensor". Certain Kv channels are rich in cysteine, and respond to the local redox environment, tending to open when oxidized and close when reduced. Candidate sensors vary the PASMC redox potential in proportion to O2. These include Nicotinamide Adenine Dinucleotide Phosphate Oxidase, (NADPH oxidase) and the cytosolic ratio of reduced/oxidized redox couples (i.e. glutathione GSH/GSSG), as controlled by electron flux in the mitochondrial electron transport chain (ETC). Using a mouse that lacks the gp91phox component of NADPH oxidase, we have recently shown that loss of the gp91phox-containing NADPH oxidase as a source of activated oxygen species does not impair HPV. However, inhibition of complex 1 of the mitochondrial electron transport chain mimics hypoxia in that it inhibits IK, reduces the production of activated O2 species and causes vasoconstriction. We hypothesize that a redox O2 sensor, perhaps in the mitochondrion, senses O2 through changes in the accumulation of freely diffusible electron donors. Changes in the ratio of reduced/oxidized redox couples, such as NADH/NAD+ and glutathione (GSH/GSSG) can reduce or oxidize the K+ channels, resulting in alterations of PA tone.

Hypoxic pulmonary vasoconstriction: role of voltage-gated potassium channels

Activity of voltage-gated potassium (Kv) channels controls membrane potential, which subsequently regulates cytoplasmic free calcium concentration ([Ca2+]cyt) in pulmonary artery smooth muscle cells (PASMCs). Acute hypoxia inhibits Kv channel function in PASMCs, inducing membrane depolarization and a rise in [Ca2+ ]cyt that triggers vasoconstriction. Prolonged hypoxia inhibits expression of Kv channels and reduces Kv channel currents in PASMCs. The consequent membrane depolarization raises [Ca2+]cyt, thus stimulating PASMC proliferation. The present review discusses recent evidence for the involvement of Kv channels in initiation of hypoxic pulmonary vasoconstriction and in chronic hypoxia-induced pulmonary hypertension.

ET(A) receptors are the primary mediators of myofilament calcium sensitization induced by ET-1 in rat pulmonary artery smooth muscle: a tyrosine kinase independent pathway

We have investigated the possibility that ET-1 can induce an increase in myofilament calcium sensitivity in pulmonary artery smooth muscle. Arterial rings were permeabilized using alpha-toxin (120 microg ml(-1)), in the presence of A23187 (10 microM) to 'knock out' Ca2+ stores, and pre-constricted with pCa 6.8 (buffered with 10 mM EGTA). In the presence of this fixed Ca2+ concentration, 1 microM ET-1 induced a sustained, reversible constriction of 0.15 mN. Pulmonary arterial rings were freeze-clamped at the peak of the induced constriction (time matched). Subsequent densitometric analysis revealed that ET-1 (1 microM) increased the level of phosphorylated myosin light chains by 34% compared to an 11% increase in the presence of pCa 6.8 alone. In contrast to ET-1, the selective ET(B) receptor agonist Sarafotoxin S6C (100 nM) failed to induce a significant constriction. The constriction induced by 1 microM ET-1 was reversibly inhibited when the preparation was preincubated (15 min) with the ETA receptor antagonist BQ 123 (100 microM). The constriction measured 0.13 mN in the absence and 0.07 mN in the presence of 100 microM BQ 123. In contrast, the constriction induced by 1 microM ET-1 measured 0.19 mN in the absence and 0.175 mN following a 15 min pre-incubation with the ET(B) antagonist BQ 788 (100 microM). The constriction induced by 1 microM ET-1 measured 0.14 mN in the presence and 0.13 mN following pre-incubation with the tyrosine kinase inhibitor Tyrphostin A23 (100 microM). We conclude that ET-1 induced an increase in myofilament calcium sensitivity in rat pulmonary arteries via the activation of ET(A) receptors and by a mechanism(s) independent of tyrosine kinase.

Signaling mechanisms underlying the vascular myogenic response

The vascular myogenic response refers to the acute reaction of a blood vessel to a change in transmural pressure. This response is critically important for the development of resting vascular tone, upon which other control mechanisms exert vasodilator and vasoconstrictor influences. The purpose of this review is to summarize and synthesize information regarding the cellular mechanism(s) underlying the myogenic response in blood vessels, with particular emphasis on arterioles. When necessary, experiments performed on larger blood vessels, visceral smooth muscle, and even striated muscle are cited. Mechanical aspects of myogenic behavior are discussed first, followed by electromechanical coupling mechanisms. Next, mechanotransduction by membrane-bound enzymes and involvement of second messengers, including calcium, are discussed. After this, the roles of the extracellular matrix, integrins, and the smooth muscle cytoskeleton are reviewed, with emphasis on short-term signaling mechanisms. Finally, suggestions are offered for possible future studies.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

Oxygen-sensing potassium currents in pulmonary artery

The pulmonary vasculature is sensitive to the relative components of the respiratory gases and will vasoconstrict in response to decreased oxygen (O2) levels. This hypoxic pulmonary vasoconstriction (HPV) controls pulmonary blood flow in the fetus and serves to maximize ventilation perfusion matching in the adult lung. The exact mechanism of HPV is not fully understood but it appears to involve direct effects on both the endothelium and smooth muscle cells within the vessel wall. There is growing evidence to suggest that hypoxia mediates vasoconstriction, at least in part through the inhibition of outward potassium (K+) current in smooth muscle. A number of K+ currents present in the pulmonary vasculature have been shown to be sensitive to O2, with hypoxia acting to inhibit these currents in the majority of cases. Differences in the expression of these O2-sensitive K+ channels may explain regional and generic variations observed in the HPV response. The mechanism by which these K+ channels sense changes in O2 levels may involve changes in the cellular redox state, oxidative phosphorylation or a direct effect on the channel protein itself.

Contribution of Ca2+-activated K+ channels and non-selective cation channels to membrane potential of pulmonary arterial smooth muscle cells of the rabbit

1. Using the perforated patch-clamp or whole-cell clamp technique, we investigated the contribution of Ca2+-activated K+ current (IK(Ca)) and non-selective cation currents (INSC) to the membrane potential in small pulmonary arterial smooth muscle cells of the rabbit. 2. The resting membrane potential (Vm) was -39.2 +/- 0.9 mV (n = 72). It did not stay at a constant level, but hyperpolarized irregularly, showing spontaneous transient hyperpolarizations (STHPs). The mean frequency and amplitude of the STHPs was 5.6 +/- 1. 1 Hz and -7.7 +/- 0.7 mV (n = 12), respectively. In the voltage-clamp mode, spontaneous transient outward currents (STOCs) were recorded with similar frequency and irregularity. 3. Intracellular application of BAPTA or extracellular application of TEA or charybdotoxin suppressed both the STHPs and STOCs. The depletion of intracellular Ca2+ stores by caffeine or ryanodine, and the removal of extracellular Ca2+ also abolished STHPs and STOCs. 4. Replacement of extracellular Na+ with NMDG+ caused hyperpolarization Vm of without affecting STHPs. Removal of extracellular Ca2+ induced a marked depolarization of Vm along with the disappearance of STHPs. 5. The ionic nature of the background inward current was identified. The permeability ratio of K+ : Cs+ : Na+ : Li+ was 1.7 : 1.3 : 1 : 0. 9, indicating that it is a non-selective cation current (INSC). The reversal potential of this current in control conditions was calculated to be -13.9 mV. The current was blocked by millimolar concentrations of extracellular Ca2+ and Mg2+. 6. From these results, it was concluded that (i) hyperpolarizing currents are mainly contributed by Ca2+-activated K+ (KCa) channels, and thus STOCs result in transient membrane hyperpolarization, and (ii) depolarizing currents are carried through NSC channels.

Potassium ion channels and human disease: phenotypes to drug targets?

A significant difficulty faced by the pharmaceutical industry is the initial identification and selection of macromolecular targets upon which de novo drug discovery programs can be initiated. A drug target should have several characteristics: known biological function; robust assay systems for in vitro characterization and high-throughput screening; and be specifically modified by and accessible to small molecular weight compounds in vivo. Ion channels have many of these attributes and can be viewed as suitable targets for small molecule drugs. Potassium (K+) ion channels form a large and diverse gene family responsible for critical functions in numerous cell types, tissues and organs. Recent discoveries, facilitated by genomics technologies combined with advanced biophysical characterization methods, have identified novel K+ channels that are involved in important physiologic processes, or mutated in human inherited disease. These findings, coupled with a rapidly growing body of information regarding modulatory channel subunits and high resolution channel structures, are providing the critical information necessary for validation of K+ channels as drug targets.